The present invention relates to a streamlined low-drag, high strength towline for towing underwater devices at depths and speeds heretofore unattainable.
High drag forces on towed devices tend to cause the device to stream out near the water surface behind the towing vessel unless depressing forces are applied to submerge the device. These depressing forces are usually applied by means of flaps and control surfaces on the towed devices, wherein the resultant depressing forces and drag forces on the towed devices are carried as tension forces in the towing cables or faired towlines that extend between the ship and the towed devices.
Although steel tow cables have been widely used to tow various submerged devices at shallow depths and low towing speeds, the increased towing speed and depth requirements for many modern towed systems has necessitated the use of faired towlines which are stronger and less ponderous than steel towing cables. For example, when sonar devices are used at great towing depths and at fast towing speeds high tension forces are produced which generally necessitate the use of stronger cables. However, since recent strength improvements in steel fiber materials for steel towing cables have been minimal, the usual recourse is to enlarge the cross-sectional size of the steel cable to accomodate the higher tension forces. But, since drag is a function of the frontal area of the towline and since the increased weight of the cables accordingly increases the tension forces in the cable, a practical length limit is soon reached where conventional steel cables become unduly ponderous for towing submerged devices at great depths and high speeds.
Drag forces on towlines also depend upon the shape of the towline and, to reduce drag forces encountered with round towing cables, streamlined fairing segments have been developed for attachment to the cables. These fairing segments normally have a streamlined profile with a tapered trailing edge portion to provide smooth fluid flow therearound. However, when such segmented fairing elements are attached to the cable a complex towing structure is produced, which not only may create undesirable turbulence and noise, but also may increase storage and handling problems as a consequence of the bulky cable, the segmented fairing elements and numerous connecting elements.
An improved form of a towline has been developed which incorporates the cable or tension member and the fairing segments into an integrated, unitary faired towline or fairing which is devoid of joints, fasteners and fairing segments that tend to increase fluid turbulence, drag, and noise. By virtue of a rounded leading edge and a tapered trailing edge to minimize drag and noise, the integrated fairing is generally non-ventilating, non-cavitating and hydrodynamically efficient. Examples of streamlined, unitary fairings are generally represented by U.S. Pat. Nos. 3,227,398; 3,304,364; 3,352,274; 3,433,020; 3,611,976; and 3,613,627. In general these fairings are composite members which include a leading edge portion utilized as a strength member and a tapered trailing edge portion. However, attempts to use some of these fairings at preselected depths and towing speeds, and at a predetermined orientation beneath the towing vessel have often been unsuccessful due to "kiting" instabilities and erratic deflection of the fairing during towing operations. For example, towing tests with fairings of the type shown in U.S. Pat. No. 3,613,627 have shown the fairing to be susceptible to hydrodynamic and mechanical instabilities as well as shape asymmetries that produce excessive towline kiting. Preliminary evaluation of this integrated fairing design indicates that kiting of the fairing results from a failure of the bond at the interface between the leading edge strength member and the trailing edge portion of the fairing. Such interface failure along that length of the fairing presumably causes irregular lateral displacement of the tapered trailing edge portion relative to the strength member to produce a longitudinal shape asymmetry in the fairing. Shape asymmetry occuring in a portion of the fairing causes unbalanced hydrodynamic forces thereabout which makes it especially difficult to maintain the fairing at a predetermined orientation during towing operations.